Polyurethane-based heterogeneous catalysts with highly dispersed Rh single sites for reductive hydroformylation of olefins

Atomically Dispersed Manganese Lewis Acid Sites Catalyze Electrohydrogenation of Nitrogen to Ammonia

Constructing and controlling of highly dispersed metallic sites for catalysis

Facile synthesis of porous Fe-doped g-C3N4 with highly dispersed Fe sites as robust catalysts for dinitro butyl phenol degradation by peroxymonosulfate activation

The catalytic partial oxidation (CPO) of methane over precious metal catalyst has been shown to be an attractive way to obtain syngas (CO and H2) or H2 which can be converted to clean fuels by Fischer–Tropsch synthesis or employed in fuel cells. However, the presence of sulphur bearing compounds naturally occurring in the fuel, or added as odorants to pipe-line natural gas (approximately up to 10 ppm), can have a detrimental effect on the CPO activity. In this work the effect of sulphur addition on the catalytic partial oxidation (CPO) of methane in the low to moderate temperature regime (300-800 °C) and under self-sustained high temperature (>800 °C) condition was investigated on Rh-based catalysts supported on either La2O3 or SiO2 stabilised γ-Al2O3. Based on the results of catalytic activity measurements and in-situ FT-IR/DRIFT spectroscopic characterisation, as well as TPR/TPD studies, it has been shown that the presence of sulphur can severely suppress the formation of synthesis gas by inhibiting the steam reforming (SR) reactions during the CPO of methane. It was demonstrated that the support material plays a crucial role in the CPO of methane in the low to moderate temperature regime. In the presence of a sulphating support such as La2O3-Al2O3 the partial oxidation reaction was much less inhibited than a less sulphating support such as SiO2-Al2O3. The sulphating support acts as a sulphur storage reservoir, which minimises the poison from adsorbing on or near the active Rh sites where reactions take place. However under the typical operating conditions of methane CPO i.e. at high temperatures and short contact times over structured reactors, sulphur in the feed inhibits the SR reaction by directly poisoning the active Rh sites thus preventing the sulphur storage capacity of the support from showing any beneficial effect on the S-tolerance. Both steady state and transient operation of the CPO reactor were investigated particularly with regards to poisoning/regeneration cycles and low temperature light-off phase. The analysis of products distribution in the effluent and heat balance demonstrated that sulphur reversibly adsorbed on Rh selectively inhibits the SR reaction path to syn-gas production. The extent of SR inhibition is greater when operating in air and diminishes at lower CH4/O2 feed ratios. The poisoning effect was also shown to be independent from the type of sulphur bearing compound and only indirectly affected by the type of catalyst support (La2O3 or SiO2 stabilised alumina) through the value of Rh dispersion. In fact by using in situ DRIFTS experiments of adsorbed CO at room temperature it was found that sulphur acts as a selective poison by preferentially adsorbing on smaller well dispersed Rh crystallites whilst larger metallic Rh sites are mostly unaffected. The adsorption of CO at room temperature before and after S poisoning is schematically represented below. Partial substitution of Rh/La-Al2O3 monolith catalysts with either Pt or Pd did not influence the way S adsorbs on highly dispersed Rh sites. Pd was found to have a detrimental effect on the overall catalytic activity and to be ineffective at improving the S-tolerance. On the other hand the partial substitution of Rh with Pt reduced the detrimental impact of S, which strongly inhibits the SR reaction on dispersed Rh sites but has a much smaller impact on Pt active sites. The improved tolerance of the bimetallic Rh-Pt catalyst against sulphur is due to its higher operating temperature related to the high oxidation activity of Pt which facilitates sulphur desorption from the catalyst and reduces its accumulation.

Support functionalization as an approach for modifying activation entropies of catalytic reactions on atomically dispersed metal sites

The hydroformylation of olefins is one of the most important homogeneously catalyzed processes in industry to produce bulk chemicals. Despite the high catalytic activities and selectivity’s using rhodium-based homogeneous hydroformylation catalysts, catalyst recovery and recycling from the reaction mixture remain a challenging topic on a process level. Therefore, technical solutions involving alternate approaches with heterogeneous catalysts for the conversion of olefins into aldehydes have been considered and research activities have addressed the synthesis and development of heterogeneous rhodium-based hydroformylation catalysts. Different strategies were pursued by different groups of authors, such as the deposition of molecular rhodium complexes, metallic rhodium nanoparticles and single-atom catalysts on a solid support as well as rhodium complexes present in supported liquids. An overview of the recent developments made in the area of the heterogenization of homogeneous rhodium catalysts and their application in the hydroformylation of short-chain olefins is given. A special focus is laid on the mechanistic understanding of the heterogeneously catalyzed reactions at a molecular level in order to provide a guide for the future design of rhodium-based heterogeneous hydroformylation catalysts.

Construction of highly dispersed iron active sites for efficient catalytic ozonation of bisphenol A

Highly efficient CeO2-supported noble-metal catalysts: From single atoms to nanoclusters

A facile sulfur-assisted method to synthesize porous alveolate Fe/g-C3N4 catalysts with ultra-small cluster and atomically dispersed Fe sites

Influence of the framework on the catalytic performance of Rh-supported Zr-MOFs in the hydroformylation of n-alkenes

Hydroformylation of olefins is one of the most important industrial processes for aldehyde production. Therein, the leaching of active metals for heterogeneous catalysts is an important issue in the hydroformylation reaction, particularly for higher olefins to produce higher alcohols. Here, different Rh/ZnO catalysts with diverse ZnO as a support were investigated and a home-made ZnO50 support was selected to prepare the Rh/ZnO50@ZIF-8 core–shell structure catalyst, which was synthesized by the growth of ZIF-8 with ZnO50 as the sacrificed template to afford Zn source. Compared with the Rh/ZnO50 catalyst, the Rh/ZnO50@ZIF-8 catalyst demonstrated a better cyclic stability in the hydroformylation of 1-dodecene. Combining the experiment and characterization results, it was concluded that the ZIF-8 shell on the Rh/ZnO50 catalyst effectively prevented the leaching of metal Rh into the reaction solution. Moreover, the Rh/ZnO50@ZIF-8 catalyst exhibited good universality for other higher olefins. This work provides a useful guideline for immobilizing the active species in heterogeneous catalysts for the hydroformylation reaction.

Hydroformylation of alkenes using heterogeneous catalyst prepared by intercalation of HRh(CO)(TPPTS) 3 complex in hydrotalcite

Fe-N/C single-atom catalysts with high density of Fe-Nx sites toward peroxymonosulfate activation for high-efficient oxidation of bisphenol A: Electron-transfer mechanism

Application of heterogeneous catalysts in olefin hydroformylation

As a new concept in catalysis, single‐atom catalyst (SAC) is becoming one of the hot topics in both homo‐ and heterogeneous catalysis, owing to its exactly identified active sites, unique electronic structure, and robust stability, selectivity, and activity in catalysis. Herein, we review the structural and electronic properties of SACs and summarize the theoretical and experimental results on a series of iron oxide–supported SACs of M 1 /FeO x (M = Pt, Ir, Au, Ni). We discuss the electronic nature of the high reactivity of SACs in catalyzing various important chemical reactions, including CO oxidation, the preferential oxidation of CO in H 2 (PROX), water gas shift (WGS) reactions, and chemoselective hydrogenation. As an extension of the SAC concept, two new types of SACs are also discussed, including singly dispersed bimetallic sites (SBMSs) of Rh 1 Co 3 on CoO support that have been shown to exhibit prominent catalytic activity for NO reduction by CO and the so‐called dynamic single‐atom catalysts (DSACs), where the dynamically formed transient monatomic species on supported gold nanoparticles are found to be the actual active sites for CO oxidation under reaction conditions. We emphasize that engineering the oxidation states of the supported transition metals is the key to achieve a high catalytic reactivity and selectivity of SACs. Finally, we summarize our understanding of the nature of SACs and provide a perspective viewpoint on the future development of SAC as a bridge of homogenous and heterogeneous catalysis.